Dithiophosphates have long been known as vulcanization accelerators for carbon black-filled diene rubbers (see for example Helfried Ehrend, “Über die Wirkung von Thiophosphaten in Vernetzungssystemen von Dienkautschuke” Gummi, Asbest und Kunststoffe 1977, 30, 68 to 71 and EP 0832920 (Bridgestone). The use of dithiophosphates as a vulcanization accelerator in activated silica-containing rubber mixtures is not described therein, however. All examples work with carbon black-filled rubber mixtures.
There are clear differences between carbon black and silica as a filler in rubber mixtures since light-colored fillers, such as silica, in contrast to carbon black, have a tendency to agglomerate in the polymer matrix. Carbon black and silica also exert different influences on a given vulcanization/acceleration system (see for example S. Wolff. et al., Silica-Based Tread Compounds: Background and Performance, presented at the Tiretech Conference 1993).
Silica-filled tire compositions are of particular interest as they lead to reduced rolling resistance without the adhesion and durability of the tires being impaired. The result is lower fuel consumption.
WO 00/05301 (EP 1102812 B1 and DE 69903563 T2 (Michelin)) describes the use of zinc dithiophosphates in combination with guanidine derivatives as the coupling activators in the activation of light-colored fillers with polysulphide-containing alkoxysilanes in order to thus considerably reduced the quantity of alkoxysilane used. However, the use of guanidine derivatives is disadvantageous in the presence of natural rubbers, which rubbers are used in particular in the production of winter tires, as a high temperatures they lead to destabilizatoin of the system (reversion, chain breakdown). Only sulphenamides are used as the vulcanization accelerator. Use of the zinc dithiophisphates as the vulcanization accelerator is not disclosed.
In the present invention, it was suprisingly discovered that the use of dithiophosphates as the vulcanization accelerator, in combination with at least one further vulcanization accelerator selected from a sulphenamide compound or a thiazole compound, lead to the formation of extremely reversion-stable silica-containing vulcanized rubber mixtures. The process is suitable in particular for solution SBR (S-SBR)-containing or S-SBR/NR-containing rubber mixtures. Compared with EP 1102812 the process does not require any coupling activators during filler activation and, in particular, no guanidine derivatives with a particularly damaging effect on NR-containing rubber mixtures need to be used. The process of the present invention is thus particularly suitable for producing reversion-stable NR-containing rubber mixtures which are suitable for producing winter tires. The process according to the present invention is also suitable for producing S-SBR/BR-containing silica-containing rubber mixtures which are particularly suitable as summer tires.